Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are introduced to the environment typically during synthesis and incorporation into final products, consumer usage, and disposal via wastewater into the receiving streams. Conventional water treatment has limitations in effectively removing PFAS from source waters due to the compounds’ complex physico-chemical properties. High-pressure membranes can serve as effective barriers to selected PFAS; however, membrane processes are energy-intensive, and membrane fouling is a major drawback in high-pressure membrane operations. Ultrafiltration (UF) is a low-pressure membrane that can save significant operating costs compared to the high-pressure membranes; however, UF has not been investigated for PFAS removal likely due to its large pore size.  The objective of this study was to modify a UF membrane via layer-by-layer (LbL) deposition of polyelectrolytes (PE) and determine its efficiency in removing perfluorooctanoic Acid (PFOA) and perfluorooctanesulfonic acid (PFOS)—the two most commonly targeted PFAS—from water. A fluidic method was followed to conformally deposit polyacrylic acid (PAA) and polyallylamine hydrochloride (PAH) on a commercially available UF membrane. PFOS and PFOA removals by the unmodified and modified UF (PAA/PAH-UF) membranes were determined using a bench-scale flat-sheet membrane experimental setup. The effects of water matrices on PFOS/A removal by the modified membrane were investigated as well.  The PAA/PAH-UF membrane became “tighter” as demonstrated by an approximately 9% reduction in membrane porosity. Following the LbL coating, the molecular weight cut-off (MWCO) of the membrane was reduced by approximately 49%. As a result, the removals of PFOA and PFOS by the PAA/PAH-UF membrane were 29% and 31% higher, respectively, when compared to the unmodified membrane. The enhanced removals of PFOA and PFOS were observed mainly due to size exclusion. The presence of cations (Ca<sup>2+</sup> and Mg<sup>2+</sup>) and humic acids (HA) in the water matrix resulted in approximately 19% higher removal of PFOA; however, when cation concentration is increased further, no additional removal of PFOA was observed. The presence of HA and cations had a significant effect on PFOS removal (14% higher) by the PAA/PAH-UF membrane only when cation concentrations were increased. This signifies that the effect of water matrix components must be investigated when targeting the removal of PFOS and PFOA using PE modified UF membranes.

Removal of perfluorooctanoic acid and perfluorooctanesulfonic acid using polyelectrolyte modified ultrafiltration membrane

ACS Fall 2021

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